Device for storage of images of invisible radiation



Sept. I3, 1955 E E, SHELDON 2,77,971

DEVICE FOR STORAGE OF IMAGES OF INVISIBLE RADIATION Filed March 30. 1949 5 Sheets-Sheet l Sept. 13, 1955 E. E. SHELDON DEVICE FOR STORAGE OF IMAGES OF INVISIBLE RADIATION Filed MarCh 50, 1949 3 Sheets-Sheet 2 I N V EN TOR.

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IN V EN TOR:

' United States Patent 0 M DEVICE FOR STORAGE F IMAGES OF INVISIBLE RADIATION Edward Emanuel Sheldon, New York, N. Y.

Application March 30, 1949, Serial No. 84,326

9 Claims. (Cl. 313-65) This invention relates to a method and device for intensifying and storing images of invisible radiations, and refers more particularly to a method and device for intensifying and storing invisible radiations, which term is meant to include X-rays, gamma rays and the like, and also for images formed by irradiation by beams of atom particles, such as electrons or neutrons. This application contains subject matter common to that disclosed in my U. S. Patent No. 2,525,832, filed February 20, 1946, and No. 2,555,423, led on April 16, 1947.

One primary objective of this invention is to provide a method and device to produce intensified images for examination. This intensification will make it possible to overcome the ineiiiciency of the present uoroscopic examination. At present, illumination of the X-ray fluoroscopic image is of the order of 0.001-001 millilambert. At this level the human eye has to rely exclusively on scotopic (dark adaptation) vision which is characterized by tremendous loss of normal visual acuity in reference both to the detail and contrast.

Another object of this invention is to make it possible to prolong the iluoroscopic examination, since it will reduce markedly the strength of radiation affecting the patients body. Conversely, the exposure time or energy necessary for examination using an invisible radiation may be considerably reduced.

Another object of this invention is to provide a method and device to produce sharper and more contrasting images of invisible radiations than it was possible heretofore.

Another purpose of this invention is to provide the possibility of storing the invisible images and inspecting them`for a desired length of time when wanted.

The present intensifying devices concerned with reproduction of X-ray images are completely unsatisfactory, because at low levels'of iiuorescent illumination, such as We are dealing with, there is not enough of X-ray photons to be absorbed by iiuorescent or photoelectric screens used in such devices. Therefore the original X-ray image can be reproduced by them only with a considerable loss of information. It is well known that the lack of sufficient number of X-ray quanta cannot be remedied by the increase of intensity of X-ray radiation, as it will result in damage to the patients body. This basic deticiency of the X-ray examination was overcome in my invention by using an X-ray exposure of a strong intensity but of a short duration, and storing the invisible X-ray image for subsequent inspection for the desired length of time without any need of maintaining the X-ray irradiation. T he X-ray beam, therefore, can be shut oit while reading the stored X-ray image and in this Way the total X-ray exposure received by the patient is not increased, in spite of using bursts of a great X-ray intensity. The storage of radar signals is well known in the art as evidenced by U. S. Patent No. 2,451,005 to P. K. Weimer, and other patents. The novelty of my invention consists of storing the images of invisible radiation and not only signals, and what is even t2,7 l 7 ,97 l Patented Sept. 13, 1955 more important, of storing simultaneously the total images instead of breaking them up into minute point images by scanning, in order to be able to store them. Further deficiency of the present storage systems, is that the Stored image can be reproduced with a good definition only if they are not intensified in the process of reproduction. In my invention, intensification of stored images is accomplished without sacrificing detail and contrast of the stored image. This feature is of a great importance especially in X-ray examinations in which, without intensification of the order of 1000 the eye is confined to so-called scotopic vision, at which it is not able to perceive definition and contrast of the fluorescent X-ray image.

The purposes of my invention were accomplished by converting the invisible radiation images into photoelectron images by using the composite photo-cathode suitable for the particular kind of radiation applied, which photo-cathodes are described in detail in my co-pending application Serial No. 59,661 filed November 5, 1948, now U. S. Patent 2,603,757 issued July l5, 1952. In case of dealing with X-ray radiation, said composite photo-cathode consists of light-refiecting layer, uorescent layer, separating layer and photo-emissive layer. In case infrared radiation is used, such a composite photo-cathode has infrared transmitting visible light reiiecting layer, such as gold, and infrared sensitive fluorescent layer, separating layer and photoemissive layer. Instead of composite photo-cathode in some instances a simple photo-cathode consisting of a layer emitting electrons under the inuence of radiation applied and of a backing plate, may be used. The photoelectron image emitted from the photoemissive layer, and having the pattern of invisible radiation image is intensified by acceleration, demagniiication and if necessary by secondary emission, and is focused by means of magnetic or electrostatic fields on the image storage target of a per forated type. A strong broad beam of electrons from another electron source disposed in the same vacuum tube is projected on that target covering simultaneously all points of the target. The photoelectron image deposited on the target controls the passage of said strong broad electron beam through the perforated target acting in a similar way to a grid in electronic tube. The transmitted electron beam is therefore modulated according to the pattern of the photo-electron image and is focused on the viewing uorescent or other electronreactive screen positioned in the same vacuum tube. The transmitted electron beam will reproduce the total image simultaneously and the reproduced image will be obviously greatly intensified as compared with the original image which was the primary objective of my invention. In view ofthe fact that the photoelectron image is deposited on the image storage target which is of dielectric or semi-conductor type, it will persist there for a long time. During all this time it will be able to control the strong broad electron beam which produces the intensiiied final image. In this way the image of invisible radiation can be stored and read when it is desired, which was another purpose of this invention.

The invention will appear more clearly from the following detailed description when taken in connection with the accompanying drawings showing by way of example only preferred embodiments -of the inventive idea.

In the drawings:

Fig. l represents a schematic sectional view of the image storage tube;

Fig. 2 represents a modiiation of the image storage tube;

Fig. 3 represents a modification of the composite photocathode in the image storage tube;

Fig. 4 represents a modification of the storage target;

Fig. 5 represents a modification of the image storage tube using cathode-ray gun;

Fig. 5a shows an image storage tube in combination with optical means;

Fig. 5b shows a modification of the image storage tube having optical means;

Fig. 6 represents a modification of the image storage tube with cathode-ray gun.

Reference will now be made to Fig. 1 illustrating the novel storage tube. The face 9a of the image tube 1 must be of material transparent to the type of the depicting radiation to be used. Inside of the face of the tube there is a composite screen 13 comprising a very thin layer 9 which is transparent to the radiation used, which refiects visible light and which prevents the loss of the light from the adjacent fluorescent layer 10, the fluorescent layer 10, very thin transparent layer 11 and photoemissive layer 12. The refiecting layer 9 may be of aluminum or silver. The fiuorescent and photoemissive layer should be correlated so that under the influence of the depicting radiation there is obtained a maximum output of photoemission. More particularly the fluorescent layer should be composed of a material having its greatest sensitivity to the type of radiation to be used and the photoemissive material, likewise, should have its maximum sensitivity to the wave-length emitted by the fluorescent layer. Fluorescent substances to be used are zinc silicates, zinc sulphides, zinc oxide, BaPtSO4 or organic phosphors, such as anthracene or naphthalene with or without activators. The satisfactory photoemissive material will be in case of depicting radiation of infrared wave-length caesium oxide activated by silver. In case of depicting radiation of short wave-length such as X- rays 38 or gamma rays, the best photoemissive surface would be of caesium, potassium o-r lithium on antimony or bismuth. The very thin transparent separating layer 11 may be of mica, silicates, quartz, or of a suitable transparent plastic. The invisible X-ray image of the examined body 14 is converted into a fiuorescent image in the fluorescent layer and then into photoelectron image in the photoemissive layer 12. The photoelectron image is accelerated and focused by the magnetic or electrostatic fields 18 and 18a on the composite target 16. The focusing and accelerating fields are not indicated in detail as they are well known in the art and will only serve to complicate the drawings. Sometimes it is better to demagnify the photoelectron image electronoptically before projecting it on said target. This can be done by the use of electron lenses 19. The photoelectron image is focused on the composite target 16 with velocity causing secondary emission from the target at the ratio greater than unity (S greater than l). The secondary electrons emitted from the dielectric or semiconductive layer 4 of said target are drawn away by the adjacent mesh screen 20. In this way the photoelectron image is deposited as a positive charge image on the layer 4. It is obvious that photoelectron image can be also focused on the composite target with velocity at which secondary electron emission is smaller than unity (S smaller than l). The resulting charge image will then be a negative one. ln such a case, the mesh screen 20 may be omitted. The composite target 16 consists of transparent metallic backing plate 2, photoemissive layer 3 and perforated dielectric layer 4. A strong source of invisible light such as ultraviolet or infrared 6, is projected on said composite target. Under the influence of invisible light a strong broad beam of photoelectrons is emitted from the photoemissive layer 3. This beam has to pass through the perforated dielectric layer 4. This passage is modulated by the charge image deposited on the opposite side of said dielectric layer by the action of the invisible image. Therefore, the beam of photoelectrons which is passing through the dielectric layer will have imprinted on it the pattern of the original invisible X-ray image. The transmitted photoelectron able to reproduce images without distortion.

beam 24 is of a much greater intensity than the original X-ray image, therefore, by converting said transmitted electron beam into a visible image in the fluorescent screen 25, a marked intensification of the original X-ray image is obtained. The fluorescent screen 25 has an electron transparent, light reflecting layer 25a, such as of aluminum, to prevent destructive back-scattering of light Instead of fluorescent screen other electron reactive surfaces may be used such as photographic films, electrolytic papers or electrographic plates. The transmitted photoelectric beam Z4 before its reproduction into visible image may be intensified by acceleration by fields 28, 2&1 and 28h, and electron-optical demagnifcation. In some instances it may be necessary to use in addition, a secondary electron emissive screen 26 of Cs-Sb, CsOAg or of insulator, like mica or glass on an electron transparent support 26a, on which photoelectron beam 24 is focused for further intensification. photoemissive layer 3 in the composite target may be of caesium, potassium or lithium on antimony when using ultraviolet source of light, or of caesium or lithium on bismuth or arsenic when using white light. In some instances it is preferable to use infrared source of light because electrons emitted by infrared are more uniform in their velocities, which is of importance for resolution of the image. In such a case, the photoemissive layer should be preferably of caesium, silver oxide. Dielectric perforated layer 4 has to be of material capable of storing the electrical charge image for a long time. Suitable materials for such purpose are mica, quartz, glass or plastics of a great electrical resistivity. These materials are dielectrics and have electrical resistivity larger than l012 ohm-cm. In particular mica has resistivity of 1018 ohm-cm. and quartz has resistivity of 1015 ohm-cm. The perforations in said layer must be of a size of 25-50 microns and must be equal and spaced uniformly throughout the whole surface of said layer in order to be The best method of producing such perforated dielectric screen is by using photoengraving method. By stippling process photography the surface of dielectric layer is covered with plurality of minute dots. Next, the dots are etched through the plate under electronic control. Another preferred method of producing perforated screens is to place a thin dielectric plate in the evacuated cathode-ray tube and to scan its surface with a sharply focused thin electron beam. By proper selection of the length of time, the electron beam remains in contact with each point of the dielectric plate and of energy of said scanning electron beam small holes may be burned through the plate in a uniform, symmetrical pattern, because of combined chemical and electrical etching action.

In some cases it may be necessary to include a very thin conducting photoelectron transparent layer 2a of wide mesh screen between the photoemissive layer 3 and dielectric layer 4, in order to apply to it a potential equalizing velocities of emitted photoelectrons.

The action of accelerating and focusing fields 13 and 28 is of sequential character. At the time of the X-ray exposure, the fields 18 and 18a are simultaneously activated. At the time of reading the stored X-ray image, when ultra violet source is in operation, the fields 28 are simultaneously activated, whereas fields 18 remain inactive. Also the action of the mesh screen 2f) is intermittent. At the time of the X-ray exposure the mesh screen 20 is connected to the ground. At the time of the ultraviolet exposure the mesh screen 20 is supplied with positive potential from the battery 29. The switch 29a serves to connect and to disconnect periodically the mesh screen 20 from the battery 29. After the X-ray image has been read, the composite target 16 has to be restored to the original condition before the next X-ray image can be stored. The dielectric layer 4 at the end of the reading has a positive or negative charge thereon according to the velocity of photoelectrons from the composite photo- The cathode 13. In o-rder to neutralize this charge, the composite photocathode is sprayed with uncontrolled X-ray beam from the X-ray tube 38 and the emitted photoelectrons are projected on the dielectric layer 4 with a velocity causing secondary electron emission therefrom of the sign neutralizing the remaining charges.

In some cases, in order to avoid electronoptical difficulties resulting from the oblique projection of the X- ray image onto storage target, the image storage tube may have the configuration shown in Figure 2. In this modification of my invention, the composite invisible radiation sensitive photocathode 13 is placed at one end of the evacuated tube 30 and the composite target 16 at the opposite end of said tube. The original X-ray image converted into photoelectron image in the compositephotocathode is accelerated and focused on the composite target by the action of magnetic or electrostatic fields 34. The transmitted photoelectron image is deflected by magnetic or electrostatic fields 33 which are not shown in detail as they are well known in the art,

onto fluorescent or other electron reactive screen 32. The screen 32 has an electron transparent light refecting backing layer 32a. The original X-ray image is reproduced therefore on said screen with a marked intensification. The mesh screen is not essential for the operation of my invention and may be omitted when the best resolution of the image is necessary,

The perforated dielectric layer 4 in the composite target 16, illustrated in Figs. l and 2, may have also another construction 42, shown in detail in Fig. 3. In this modification the dielectric layer such as for example of quartz or of a suitable plastic 4b is evaporated on a very wide mesh supporting screen 4a, so that openings in said screen are not obstructed and may function as the perforations described above. This construction is applicable not only in the image storage tube illustrated in Fig. 3 but in other modifications of the image storage tube as well. In particular the perforated storage target 48 shown in Figs. 5, 6 and 7 may have also the above described construction.

In some cases, instead of having composite photocathode 13, a photocathode 62 of photoemissive layer 63 on the X-ray transparent backing plate 64 is used. This embodiment of my invention is shown in Fig. 5a. In such a case, the X-ray image is projected on the fluorescent screen 6i) positioned outside of the image tube 65 and the resulting fluorescent image is projected by refractive or reflective optical system 61 on said photocathode.

In other cases, instead of having a composite photocathode 13, a simple electron emissive photocathode 40 of a thin layer of a heavy metal, such as lead, gold, bismuth or uranium mounted within the vacuum tube, may be used, as illustrated in Fig. 3. The photoelectron image from said photocathode is focused 0n the secondary electron emissive plate 41 and thereafter on the dielectric target 42, as described above. The plate 41 consists of a wide mesh screen 41a, a thin metallic layer such as of aluminum or magnesium 41b transmitting electrons and of a thin secondary electron emissive layer 41C, e. g. of glass, beryllium or magnesium. This arrangement allows the secondary electrons to emerge from the side of the plate opposite to the source of the primary electron beam.

Another modification of this invention is shown in Fig. 4. In this modification, instead of a composite target 16, a separate photocathode 45 and dielectric target 48, are used. The photocathode 45 consists of light transparent backing plate 46 an-d photoemissive layer 47. rThe photoemissive layer is chosen according to the radiation used, as was explained above. In case ultraviolet light is used, the preferable material will be eaesium or potassium on antimony. In case infra red light is used, the best material will be CsOAg. The broad beam of photoelectrons is released from the photocathode 45 by said light source, is focused by electrostatic or magnetic fields 49 on the perforated dielectric target 48 and is transmitted through it 24a according to the charge pattern on said dielectric target. The rest of the operation of the image storage tube is the same as it was described above. It is obvious that my invention may be used not only for X-ray images but as well for images produced by other invisible radiations Whether of undulatory or of corpuscular nature. In particular, my image storage tube may have application for the storage of infra red images. In such a case, the composite photocathode 51 has to be made sensitive to infra red rays. Such a photocathode may be of photoemissive layer 43 CsOAg on infra red transparent backing plate 44.

The invisible radiation sensitive photocathode 51 and E the photocathode 45 may be obviously disposed in many different ways in relation to the target 48, as well as to each other. All these modifications are within the scope and spirit of my invention.

Instead of using light radiation to produce a broad beam of photoelectrons, a special electron gun delivering a broad beam of electrons may be provided. This modification of my invention is shown in Figure 5. The electron gun 50 is producing a broad beam of electron of high intensity. This beam is transmitted through the perforated dielectric target 48 accor-ding to the pattern of charges deposited on it by the stored photoelectron image representing the original X-ray image. The photoelectron image before being stored may be further intensified by the cascade amplification described in detail in my patent, U. S. Patent No. 2,555,424. In this case, the photoelectron image from the composite photocathode 13 is focused on a similar composite screen 13a for further intensification and thereafter on the dielectric storage target 48.

Instead of a continuous broad beam of electrons having the diameter of the storage target, a flat ribbon type of scanning electron beam may be also used, which covers simultaneously one line of the image. The flat ribbon type of scanning beam is well known in the art, and therefore does not have to be described in detail here. The broad electron beam should be preferably of the low velocity. In some applications, however, a high velocity electron beam may be used as well. The operation of the image storage tube when using high velocity electron beam is similar as described above and is shown in Fig. 5. The photoelectron image from the composite photocathode 13 is focused by means of magnetic or electrostatic fields 53 on the target 48 and causes secondary electron emission from said target. The secondary electrons from the target 48 are led away by the mesh screen 52 connected in this phase of operation to the ground. When the stored photoelectron image is to be read, the electron gun Sti is activated. The broad electron beam is focused by magnetic or electrostatic fields 54 on the perforated target. In this phase of operation the mesh screen 52 is disconnected from the ground and the fields 53 are inactive. The broad beam of electrons due to its high velocity causes secondary electron emission from the target 43. The secondary electrons are transmitted through the perforated target 48 according to the pattern of charges deposited thereon by the electron image. The transmitted secondary electron beam having now the pattern of original X-ray image is accelerated and focused by magnetic or electrostatic fields 54a on the fluorescent viewing screen 55 having the electron transparent, light reflecting backing layer 56.

It is obvious that the composite photocathode 13 and the electron gun 50 may be disposed in many different ways. One of such modifications is shown by the way of example only in Fig. 6. In this case, the X-ray image is projected on the side of the perforated target opposite to the one facing the electron gun. This arrangement is especially suitable for the use with high velocity electron beam. The target 48 in some cases should have conducting backing layer 48a.

My invention can be also used for intensification of images of invisible radiations, Without storing them prior to their reading. In such a case, the dielectric target has to have suicient conductivity to allow the stored image charge to dissipate completely Within a desired period of time. In this case, operation of the image tube and of X-ray source has to be continuous instead of being intermittent, as was described above.

If the invisible electron image is projected first on the fluorescent screen 6i) outside of the image storage tube 66, so that the X-ray image is converted into'fiuorescent image outside of the vacuum tube, the perforated storage target has to be modified accordingly. This embodiment of my invention is shown in Fig. b. In such a case, the perforated storage target 67 will consist of a perforated photoemissive mosaic layer 68 and of a perforated dielectric layer 69 deposited on a mesh screen 70 in a similar way as shown in Fig. 3. The fluorescent X-ray image 14a will be projected by the optical system 71 on said storage target and will be converted into a photoelectron image and stored as a charge image in said photoemissive mosaic layer 68. This charge image will control the passage of a broad electron beam 72 from an uncontrolled source 73 through said perforated target, as Was explained above.

The sensitivity of invisible radiation image storage tube can be markedly increased by the use of a storage phosphor for the uorescent layer in the composite photocathode 13. The invisible X-ray image is stored in said phosphor and is released therefrom in the form of Huorescent image only after irradiation with an additional source of radiation, such as infrared. In this case, the visible light reflecting layer 9 obviously must be transparent to infrared radiation. A thin layer of gold Will be suitable for this purpose. Satisfactory phosphors for the storage of images are alkaline earth sulphides and selenides activated with cerium, samarium or europium, sulphides acivated With lead or with copper, or lanthanum oxysulphides with activators. In case image storage tube should serve for storage and detection of infrared images, the phototcathode should be irradiated with ultraviolet radiation from an extraneous source after the exposure to infrared image. In such case, the light reliecting layer 9 obviously has to be transparent to ultraviolet light. The phosphor of the fluorescent layer may be the same as described above, that is of alkaline earth sulphides and selcnides activated with cerium, samarium or europium, of sulphides activated with lead or with copper, or lanthanum oxysulphides with activators. The storage phosphors are more sensitive to electrons than to X-rays. A good way of taking advantage of greater sensitivity of storage phosphors to electrons is to use them in a fiuorescent layer in a composite screen 3.361 representing the second stage of the image section, see Fig. 5. This arrangement is advantageous because in such case the storage phosphor is excited by the electron beam having the pattern of the X-ray image instead of by the invisible X- ray image itself.

It will thus be seen that there is provided a device in which the several objects of this invention are achieved and which is Well adapted to meet the conditions of practical use.

As various possible embodiments might be made of the above invention, and as various changes might be made in the embodiment above set forth, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A storage tube having in combination uorescent means and photoelectric means for receiving an image and converting said image into a broad current of electrons representing said image, a perforated dielectric target for receiving said electron image and for storing said image, means for producing an electron beam of low velocity and means for projecting said electron beam on said perforated target for modulating transmission of said electron beam through said perforated target with said stored image.

2. A device as defined in claim l, in which said fluorescent means have the property of storing the image.

3. A device as defined in claim l, in which means for converting an image into a fiuorescent image have the property of storing said image and releasing said stored image as a light image when irradiated by another radiation.

4. A storage tube having in combination photoelectric means for receiving an image and converting said image into a broad current of electrons representing said image, a perforated dielectric target for receiving said electron image and for storing said image, means for producing an electron beam of low velocity and means for projecting said electron beam on said perforated target for modulating transmission of said electron beam through said perforated target with said stored image.

5. A device, as defined in claim 4, in which said perforated target consists of a perforated screen of dielectric material.

6. A storage tube having in combination photoelectric means for receiving and converting an image into a broad current of electrons representing said image, a perforated dielectric target having storing surface of dielectric material for receiving said electron image and for storing said image, means for producing an electron beam of low velocity, means for projecting said electron beam on said perforated target for modulating transmission of said electron beam through said perforated target with said stored image, said means for producing the electron beam also operating to discharge said stored image.

7. A storage tube having in combination photoelectric means for receiving an image and converting said image into a broad current of electrons representing said image, a perforated dielectric target for receiving said electron image and for storing said image, means for producing an electron beam of low velocity, means for projecting said electron beam on said perforated target for modulating transmission of said electron beam through said perforated target with said stored image and electron reactive image reproducing means for receiving electrons of said modulated electron beam.

8. A storage tube having in combination photoelectric means for receiving an image and converting said image into a broad current of electrons representing said image, a perforated dielectric target for receiving said electron image and for storing said image, means for producing a broad electron beam of low velocity, means for projecting said broad electron beam on said perforated target for modulating transmission of said broad electron beam through said perforated target with said stored image and electron reactive screen compriisng a light reflecting layer and fluorescent means for receiving electrons of said modulated beam.

9. A storage tube having in combination photocathode means for receiving and converting an image into a broad current of electrons representing said image, a perforated dielectric target for receiving said electron image and for storing said image, means for producing a broad electron beam of low velocity, means for projecting said broad electron beam on said perforated target for modulating transmission of said electron beam through said perforated target with said stored image, electron reactive means for receiving electrons of said modulated electron beam transmitted through said perforated target, said means for producing the electron beam also operating to discharge said stored image.

(References on following page) References Cited in the le of this patent UNITED STATES PATENTS Orvin June 14, 1938 Bitner June 14, 1938 Lubszynski May 7, 1940 Kallmann Ian. 20, 1942 Kallmann Sept. 29, 1942 Kallmann et al Mar. 14, 1944 10 Kallmann et al. Mar. 14, 1944 Chilowsky Jan. 31, 1950 Mason et al. Sept. 19, 1950 Sheldon Oct. 17, 1950 Schlesinger Dec. 5, 1950 Sheldon June 5, 1951 Hunter et al. June 5, 1951 Sheldon July 15, 1952 

